The invention relates to a mixed flow gas turbine engine of the bypass type and, more particularly, to a mixed flow gas turbine engine suitable for powering aircraft wherein the engine bypass ratio may be controlled to satisfy particular engine operating conditions.
Considerable attention has been devoted to developing a gas turbine engine with the high specific thrust characteristics of a turbojet or low bypass turbofan at supersonic speeds which can also be configured to exhibit the lower specific thrust, low noise and low fuel consumption characteristics of a high bypass turbofan at subsonic speeds in order that a mixed-mission aircraft may be developed.
To this end, modern aircraft designers have worked to develop the aircraft engine design criteria which would enable the development of a suitable mixed-mission aircraft. Several design approaches to this problem have been offered. Such prior art systems have included various concepts of retractable fans, variable area turbines, variable pitch fans, as well as more complex techniques such as those utilizing combinations of turbofan and turbojet engines in tandem or concentric flow relation. In addition to a lack of flow flexibility, these arrangements have the obvious disadvantage of being inefficient due to the dead weight associated with those engine components not used in all modes of flight.
More recent attempts at developing practical variable bypass ratio engines include the selective direction of the inlet fan stream through alternative upstream fan ducts using inverter valves. While more effective than prior attempts at achieving satisfactory mixed-mission performance, such systems have exhibited several negative characteristics. These include the addition of extra undesired length, weight and complexity to the engine.
A further disadvantage of prior art variable bypass ratio engines is that they have failed to meet desired performance goals in all modes of operation because they have insufficient flow variability to maintain satisfactory engine performance in both supersonic and subsonic flight.
One of the reasons that prior art variable bypass mixed flow engines lack a high degree of flow variability is that in order to avoid the large shock losses which result from the direct mixing of two streams of significantly different static pressures, the static pressures of the core and bypass streams must be balanced prior to mixing. In prior art mixed flow engines the static pressure of the bypass stream is controlled by setting the dynamic pressure of the bypass stream. Thus, in order to balance the core and bypass stream pressures prior to mixing, the bypass stream dynamic pressure and resultant static pressure must be maintained at a level at which the core and bypass stream static pressures are nearly equal immediately prior to mixing. However, since the engine bypass ratio which is defined as the ratio of the bypass flow to core stream flow is dependent upon the bypass stream dynamic pressure, the core stream static pressure effectively determines the bypass ratio at which prior art engines may operate, without experiencing the relatively large shock losses which result from not balancing the core and bypass stream static pressures prior to mixing. This interdependence of the core stream static pressures in the mixing region and the dynamic pressure of the bypass stream has prevented prior art variable bypass mixed flow engines from operating efficiently throughout a wide range of bypass ratios and thrust levels.
Another reason prior art mixed flow engines have not operated efficiently throughout a variable velocity range is that they experience significantly high inlet drag levels during low thrust flight. Typically, the inlet of a gas turbine engine is sized to be full at the maximum thrust of the engine. However, as engine thrust is decreased below the maximum thrust, the engine airflow demand is considerably less than the total airflow supplied to the inlet. This excess airflow to the inlet causes inlet spillage drag which significantly increases the installed fuel consumption of prior art mixed flow engines.